Transactions

Report
1DT066
DISTRIBUTED INFORMATION SYSTEM
Transactions and Concurrency
Control
1
OUTLINE
 Motivation
 Transaction
 Two
Concepts
Phase Commit
 Distributed
Transactions and Deadlocks
 Summary
2
1 MOTIVATION
What happens if a failure occurs during modification of
resources?
 Which operations have been completed?
 Which operations have not (and have to be done again)?
 In which states will the resources be?

3
1 REVISIT OF FUNDS TRANSFER EXAMPLE
Balances at t0 Acc1: 7500, Acc2: 0
Funds transfer from Acc1 to Acc2:
t0
t1
t2
t3
t4
t5
t6
t7
Time
Acc1->debit(7500):
Acc1->lock(write);
Acc1.balance=0;
Acc1->unlock(write);
Acc2->credit(7500):
Acc2->lock(write);
Acc2.balance=7500;
Acc2->unlock(write);
4
1 FUNDS TRANSFER IN CONCURRENCY
Balances at t0 Acc1: 7500, Acc2: 0
Funds transfer from Acc1
t0
t1
t2
Funds transfer to Acc2
Acc1->debit(7500):
t3
t4
t5
t6
t7
Time
Acc1->lock(write);
Acc2->credit(7500):
Acc1.balance=0;
Acc2->lock(write);
Acc1->unlock(write);
Acc2.balance=7500;
Acc2->unlock(write);
5
2 TRANSACTION CONCEPTS
1 ACID Properties
Atomicity
 Consistency
 Isolation
 Durability

2 Transaction Commit vs. Abort
3 Roles of Distributed Components
4 Flat vs. Nested Transactions
6
2.1.1 ATOMICITY



Transactions are either performed completely or no
modification is done.
Start of a transaction is a continuation point to which it
can roll back.
End of transaction is next continuation point.
7
2.1.2 CONSISTENCY
Shared resources should always be consistent.
 Inconsistent states occur during transactions:

hidden for concurrent transactions
 to be resolved before end of transaction.

Application defines consistency and is responsible for
ensuring it is maintained.
 Transactions can be aborted if they cannot resolve
inconsistencies.

8
2.1.3 ISOLATION
Each transaction accesses resources as if there were
no other concurrent transactions.
 Modifications of the transaction are not visible to
other resources before it finishes.
 Modifications of other transactions are not visible
during the transaction at all.
 Implemented through:

two-phase locking or
 optimistic concurrency control.

9
2.1.4 DURABILITY



A completed transaction is always persistent (though
values may be changed by later transactions).
Modified resources must be held on persistent storage
before transaction can complete.
Wide use of hard disks.
10
2.2 TRANSACTION COMMANDS

Begin:


Start a new transaction.
Commit:
End a transaction.
 Store changes made during transaction.
 Make changes accessible to other transactions.


Abort:
End a transaction.
 Undo all changes made during the transaction.

11
2.3 ROLES OF COMPONENTS
Distributed system components involved in
transactions can take role of:
Transactional
Client
Transactional
Server
Coordinator
12
2.3.1 COORDINATOR
Coordinator plays key role in managing transaction.
 Coordinator is the component that handles begin /
commit / abort transaction calls.
 Coordinator allocates system-wide unique transaction
identifier.
 Different transactions may have different coordinators.

13
2.3.2 TRANSACTIONAL SERVER
Every component with a resource accessed or
modified under transaction control.
 Transactional server has to know coordinator.
 Transactional server registers its participation in a
transaction with the coordinator.
 Transactional server has to implement a transaction
protocol (two-phase commit).

14
2.3.3 TRANSACTIONAL CLIENT
Only sees transactions through the transaction
coordinator.
 Invokes services from the coordinator to begin,
commit and abort transactions.
 Implementation of transactions are transparent for
the client.
 Cannot tell difference between server and
transactional server.

15
2.4 DISTRIBUTED TRANSACTIONS
(a) Flat transaction
(b) Nested transactions
M
X
T11
X
Client
T
Y
T
T1
N
T 12
T
T
21
T2
Client
Y
P
Z
T
22
16
2.4 FLAT TRANSACTIONS
Begin
Trans.
Commit
Flat Transaction
Begin
Trans.
Crash
Flat Transaction
Rollbac
k
Begin
Trans.
Abort
Flat Transaction
Rollbac
k
17
2.4 NESTED TRANSACTIONS
Begin
Trans.
Commit
Main Transaction
Call
Begin
Trans.
Call
Commit
Begin
Trans.
Commit
Call
Begin
Trans.
Commit
18
3 TWO-PHASE COMMIT

Multiple autonomous distributed servers:


For a commit, all transactional servers have to be able to commit.
If a single transactional server cannot commit its changes every
server has to abort.
Single phase protocol is insufficient.
 Two phases are needed:

Phase one: Voting
 Phase two: Completion.

19
3.1 PHASE ONE
Called the voting phase.
 Coordinator asks all servers if they are able (and
willing) to commit.
 Servers reply:

Yes: it will commit if asked, but does not yet know if it is
actually going to commit.
 No: it immediately aborts its operations.


Hence, servers can unilaterally abort but not
unilaterally commit a transaction.
20
3.1 PHASE TWO
Called the completion phase.
 Co-ordinator collates all votes, including its own, and
decides to

commit if everyone voted ‘Yes’.
 abort if anyone voted ‘No’.


All voters that voted ‘Yes’ are sent



‘DoCommit’ if transaction is to be committed.
Otherwise ‘Abort'.
Servers acknowledge DoCommit once they have
committed.
21
3.1 SERVER UNCERTAINTY
Period when a server must be able to commit, but
does not yet know if has to.
 This period is known as server uncertainty.
 Usually short (time needed for coordinator to receive
and process votes).
 However, failures can lengthen this process, which
may cause problems.

22
3.2 RECOVERY IN TWO-PHASE COMMIT
Failures prior to start of 2PC results in abort.
 Coordinator failure prior to transmitting commit messages
results in abort.
 After this point, coordinator will retransmit all commit
messages on restart.
 If server fails prior to voting, it aborts.
 If it fails after voting, it sends GetDecision.
 If it fails after committing it (re)sends HaveCommitted
message.

23
3.2 COMPLEXITY
Assuming N participating servers:
 (N-1) Voting requests from coordinator to servers.
 (N-1) Votes from servers to coordinator.
 At most (N-1) Completion requests from coordinator to servers.
 (When commit) (N-1) acknowledgement from servers to
coordinator.
 Hence, complexity of requests is linear in the number of
participating servers.
24
3.3 COMMITTING NESTED TRANSACTIONS

Cannot use same mechanism to commit nested
transactions as:
subtransactions can abort independent of parent.
 subtransactions must have made decision to commit or abort
before parent transaction.


Top level transaction needs to be able to communicate its
decision down to all subtransactions so they may react
accordingly.
25
3.3 PROVISIONAL COMMIT

Subtransactions vote either:
aborted or
 provisionally committed.

Abort is handled as normal.
 Provisional commit means that coordinator and
transactional servers are willing to commit
subtransaction but have not yet done so.

26
3.3 EXAMPLE FOR A NESTED TRANSACTION
T
11
T1
provisional commit (at X)
T
T
provisional commit (at N)
T21
provisional commit (at N)
T
provisional commit (at P)
12
T
2
abort (at M)
aborted (at Y)
22
27
3.3 INFORMATION HELD BY COORDINATORS
Coordinator of
transaction
T
T1
T2
T 11
T 12 , T 21
T 22
Child
transactions
T 1, T 2
T 11 , T 12
T 21 , T 22
Participant
Provisional
commit list
T 1, T 12
T 1, T 12
yes
yes
no (aborted)
no (aborted)
T 12 but not T 21
T 21 , T 12
no (parent aborted)T 22
Abort list
T 11 , T 2
T 11
T2
T 11
28
3.3 TWO-PHASE COMMIT FOR NESTED
TRANSACTIONS
 For
nested transactions, the top-level transaction
plays as coordinator, while participants are all the
provisionally committed subtransaction
coordinators without aborted ancestors.
 Hierarchic two-phase commit: a multi-level nested
protocol where the coordinator communicates to the
immediate child transaction coordinator in a
hierarchic fashion.
 Flat two-phase commit: the coordinator contact all
participants with provisional commit directly.
29
3.3 LOCKING AND PROVISIONAL COMMITS
Locks cannot be released after provisional commit.
 Data items remain ‘protected’ until top-level transaction
commits.
 This may reduce concurrency.
 Interactions between sibling subtransactions:

should they be prevented as they are different?
 allowed as they are part of the same transaction?


Generally they are prevented.
30
4 DISTRIBUTED TRANSACTIONS AND
DEADLOCKS
In distributed transactions, each server is responsible
for applying concurrency control to its own objects,
and all the servers jointly ensure the concurrent
transactions are performed in a serially equivalent
manner.
 This means interleavings of two transactions have to
be serially equivalent both locally at each server and
globally.

31
4.1 INTERLEAVINGS OF TWO TRANSACTIONS
T
Write (A)
U
at X
Write (B)
Read (B)
at Y
Read (A)



at Y
at X
Transaction T before Transaction U on server X
Transaction U before Transaction T on server Y
This is not serially equivalent globally since T before U in one
server and U before T in another.
32
4.1 INTERLEAVINGS OF TRANSACTIONS U, V AND W
U
d.deposit(10)
a.deposit(20)
b.withdraw(30)
V
lock D
at Z
b.deposit(10)
lock A
at X
W
lock B
at Y
c.deposit(30)
lock C
at Z
a.withdraw(20)
wait at X
wait at Y
c.withdraw(20)
wait at Z
33
4.3 DISTRIBUTED DEADLOCK
(a)
(b)
W
Held by
D
C
A
X
Z
Waits
for
W
Waits for
V
Held
by
Held by
V
U
U
Waits for
B
Held
by
Y
34
4.2 LOCAL AND GLOBAL WAIT-FOR GRAPHS
local wait-for graph
T
U
X
local wait-for graph
V
global deadlock detector
T
T
Y
U
V
• Phantom deadlock: A deadlock that is “detected” but is
not really a deadlock is called a phantom deadlock.
• E.g.: Transaction U releases an object at server X and
requests the one held by V at server Y. Assuming the
35
latter is first received.
4.3 PROBES TRANSMITTED TO DETECT DEADLOCK
W
W U  V  W
Held by
Waits for
Deadlock
detected C
A
Z
W U  V
Waits
for
Initiation
X
W U
V
U
Held by
Y
B
Waits for
36
4.3 TWO PROBES INITIATED
(a) initial situation
Waits for
V
Waits for
T
U
W
(c) detection initiated at object
requested by W
(b) detection initiated at object
requested by T
Waits
for
T
V
T
U
T
W
U
TUWV
TUW
V 
W
V
VTU
U
W
W
T
V
W
Waits
for
37
6 SUMMARY

Transaction concepts:




ACID
Transaction commands
Roles of distributed components in transactions
Two-phase commit


phase one: voting
phase two: completion
Distributed Transactions and Distributed Deadlocks
 Read Textbook Chapter 16.

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